In recent years, transition from CRT displays to liquid crystal displays has been progressing as displays for personal computers. The CRT displays use Braun tubes and the liquid crystal displays have characteristics of light weight, flat screen, and the like. It is certain that the transition from CRT displays to liquid crystal displays, or EL displays, which will be fully introduced, will be made in the future not only for the displays used for personal computers but also for the market in which the CRTs have been used, such as for television, broadcasting, commercial use, or medical field.
One of the characteristics of crystal displays is lower power consumption comparing to CRT displays. However, the majority of the power is consumed by backlight (normally, a cold-cathode tube which is similar to fluorescent tube, is used). Further, with the strong demand for high brightness, the brightness of the backlight has been increased, and the power consumption tends to be increasing.
By the way, many of current CRT displays and liquid crystal displays automatically switch to a power save mode to reduce the power consumption when there is no image signal. In the power save mode, a screen turns into black because the power of cathode-ray tube section or liquid crystal section (or backlight) is turned off when there is no image signal. By this function, the power consumption can be automatically reduced when there is no image signal. For example, the power save mode can be used to a display for personal computer because there is no signal provided from the computer to the display when key operation or mouse operation is not performed for a predetermined time.
Meanwhile, one of Japanese patent documents discloses a display having a structure in which the brightness of the liquid crystal backlight is reduced, or the gain of data of an image signal itself is reduced, by automatically recognizing a specific still image, such as color bar. The document indicates that the power consumption can be saved by using this function when the display is not in use even when the image signal is consistently input.
Because these functions are effective functions for saving the power consumption, it is thought that the similar power save control will be applied to new displays, such as EL displays, that will be introduced in the future. However, in case there is no image signal, the method of reducing the power consumption of overall display device by turning off the backlight has a problem of potentially confusing users by completely turning off the backlight because the users do not understand why the screen turned black. In addition, the method of informing users of no image signal with “No Signal” on the screen has a problem of not being able to reduce the power consumption while it can inform the user of no signal state because the backlight needs to be kept on. Under these circumstances, there has been a demand for new technique to reduce the power consumption of display devices while informing users that there is no image signal.